The charging or discharging speed of thin film transistors to pixels can be represented by a subthreshold swing, and the smaller the subthreshold swing, the faster the charging or discharging speed of thin film transistors to pixels. The subthreshold swing is defined as the change value of a gate voltage corresponding to the current change per ten times when a source-drain voltage is a constant. The subthreshold swing is associated with the electrostatic control capacity of the gate electrode of a thin film transistor over the channel, and when the electrostatic control capacity of the gate electrode over the channel is strong, the change value of the gate voltage corresponding to the current change per ten times is small, namely, the subthreshold swing is small.